[WEXT] net_device: Don't include wext bits if not required.

This patch makes the wext bits in struct net_device depend on
CONFIG_WIRELESS_EXT.

Signed-off-by: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: John W. Linville <linville@tuxdriver.com>
Signed-off-by: David S. Miller <davem@davemloft.net>